1. Field of the Invention
This invention relates to a method of producing a light-transmitting filter used for illuminance correction purposes in an exposure device adapted for producing a phosphor screen of a cathode ray tube. More particularly, it relates to a method by means of which a light-transmitting filter presenting variable light transmittance in various portions of the filter and preset distribution characteristics of transmitted light may be produced accurately and in a short time.
2. Description of the Prior Art
On the phosphor screen of a color cathode ray tube, there are arranged an extremely large number of dots or stripes of the tricolor phosphors that emit red, blue and green lights when excited by an electron beam. In preparing the screen, a phosphor slurry is applied to the inner surface of the panel of a cathode ray tube. The slurry is exposed to a light beam by an exposure device making use of the light-transmitting filter and developed for producing a completed phosphor screen.
For producing the light transmitting filter used in the exposure device, there are known various methods, such as photographic or transcription method, and a method consisting in sequentially combining small pieces of different light transmittance. In the photographic or transcription method, a master pattern consisting of the reverse of the the light transmitting filter pattern to be produced is prepared by a plotter and transferred onto a photographic dry plate having a higher gamma value. Alternatively, the step of producing the master pattern is omitted and a pattern consisting of a combination of stripes of variable widths is formed on the photographic dry plate with a higher gamma value by exposing the dry plate to a light beam whose intensity is modulated by means of an interposed rotary plate. The pattern thus formed on the dry plate is developed and fixed. A glass plate coated with a photo-resist layer is intimately applied to the photographic dry plate. After exposure, the carbon slurry is poured on the glass plate. The pattern is transferred to the carbon mass after development for providing the light transmitting filter having, the preset light transmittance characteristics. This prior-art method has the deficiency that a number of steps are required so that manufacture time can generally not be reduced. Moreover, it is not possible with the prior-art method to produce the light transmitting filter of higher accuracy because of the error caused during the steps of preparation of the master pattern, exposure of the photographic dry plate, development/fixing and transfer to and development of the carbon slurry.
There are also known methods of changing the light transmittance of the filter as by changing the stripe width or line width, or the method of changing the pitch or line width of the mesh pattern. However, with these known methods of changing the line widths, it is not possible to produce the filter with sufficient accuracy, or to avoid the problem of moire.
The recent tendency is towards using a high precision color cathode ray tube with an extremely narrow phosphor stripe interval pitch of 0.2 to 0.3 mm. Above all, a high degree of uniform whiteness is required of the high precision cathode ray tube employed in a broadcasting station for monitoring purposes. For this reason, a high-precision and high-fidelity control technique is required for the preparation of the high-precision color cathode ray tube, so that a high-precision light transmitting filter needs to be used in the exposure device. However, it is difficult with the above described conventional methods to prepare the high precision and high fidelity light-transmitting filter capable of meeting such requirement. Hence, a demand has existed for a high precision light transmitting filter.